1 files changed, 134 insertions, 116 deletions

diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 6b6c8571e23..604d823f686 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -157,16 +157,9 @@ static Snapshot HistoricSnapshot = NULL;
  * These are updated by GetSnapshotData.  We initialize them this way
  * for the convenience of TransactionIdIsInProgress: even in bootstrap
  * mode, we don't want it to say that BootstrapTransactionId is in progress.
- *
- * RecentGlobalXmin and RecentGlobalDataXmin are initialized to
- * InvalidTransactionId, to ensure that no one tries to use a stale
- * value. Readers should ensure that it has been set to something else
- * before using it.
  */
 TransactionId TransactionXmin = FirstNormalTransactionId;
 TransactionId RecentXmin = FirstNormalTransactionId;
-TransactionId RecentGlobalXmin = InvalidTransactionId;
-TransactionId RecentGlobalDataXmin = InvalidTransactionId;
 
 /* (table, ctid) => (cmin, cmax) mapping during timetravel */
 static HTAB *tuplecid_data = NULL;
@@ -581,9 +574,7 @@ SetTransactionSnapshot(Snapshot sourcesnap, VirtualTransactionId *sourcevxid,
 	 * Even though we are not going to use the snapshot it computes, we must
 	 * call GetSnapshotData, for two reasons: (1) to be sure that
 	 * CurrentSnapshotData's XID arrays have been allocated, and (2) to update
-	 * RecentXmin and RecentGlobalXmin.  (We could alternatively include those
-	 * two variables in exported snapshot files, but it seems better to have
-	 * snapshot importers compute reasonably up-to-date values for them.)
+	 * the state for GlobalVis*.
 	 */
 	CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
 
@@ -957,36 +948,6 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
 }
 
 /*
- * Get current RecentGlobalXmin value, as a FullTransactionId.
- */
-FullTransactionId
-GetFullRecentGlobalXmin(void)
-{
-	FullTransactionId nextxid_full;
-	uint32		nextxid_epoch;
-	TransactionId nextxid_xid;
-	uint32		epoch;
-
-	Assert(TransactionIdIsNormal(RecentGlobalXmin));
-
-	/*
-	 * Compute the epoch from the next XID's epoch. This relies on the fact
-	 * that RecentGlobalXmin must be within the 2 billion XID horizon from the
-	 * next XID.
-	 */
-	nextxid_full = ReadNextFullTransactionId();
-	nextxid_epoch = EpochFromFullTransactionId(nextxid_full);
-	nextxid_xid = XidFromFullTransactionId(nextxid_full);
-
-	if (RecentGlobalXmin > nextxid_xid)
-		epoch = nextxid_epoch - 1;
-	else
-		epoch = nextxid_epoch;
-
-	return FullTransactionIdFromEpochAndXid(epoch, RecentGlobalXmin);
-}
-
-/*
  * SnapshotResetXmin
  *
  * If there are no more snapshots, we can reset our PGXACT->xmin to InvalidXid.
@@ -1753,106 +1714,157 @@ GetOldSnapshotThresholdTimestamp(void)
 	return threshold_timestamp;
 }
 
-static void
+void
 SetOldSnapshotThresholdTimestamp(TimestampTz ts, TransactionId xlimit)
 {
 	SpinLockAcquire(&oldSnapshotControl->mutex_threshold);
+	Assert(oldSnapshotControl->threshold_timestamp <= ts);
+	Assert(TransactionIdPrecedesOrEquals(oldSnapshotControl->threshold_xid, xlimit));
 	oldSnapshotControl->threshold_timestamp = ts;
 	oldSnapshotControl->threshold_xid = xlimit;
 	SpinLockRelease(&oldSnapshotControl->mutex_threshold);
 }
 
 /*
+ * XXX: Magic to keep old_snapshot_threshold tests appear "working". They
+ * currently are broken, and discussion of what to do about them is
+ * ongoing. See
+ * https://www.postgresql.org/message-id/20200403001235.e6jfdll3gh2ygbuc%40alap3.anarazel.de
+ */
+void
+SnapshotTooOldMagicForTest(void)
+{
+	TimestampTz ts = GetSnapshotCurrentTimestamp();
+
+	Assert(old_snapshot_threshold == 0);
+
+	ts -= 5 * USECS_PER_SEC;
+
+	SpinLockAcquire(&oldSnapshotControl->mutex_threshold);
+	oldSnapshotControl->threshold_timestamp = ts;
+	SpinLockRelease(&oldSnapshotControl->mutex_threshold);
+}
+
+/*
+ * If there is a valid mapping for the timestamp, set *xlimitp to
+ * that. Returns whether there is such a mapping.
+ */
+static bool
+GetOldSnapshotFromTimeMapping(TimestampTz ts, TransactionId *xlimitp)
+{
+	bool		in_mapping = false;
+
+	Assert(ts == AlignTimestampToMinuteBoundary(ts));
+
+	LWLockAcquire(OldSnapshotTimeMapLock, LW_SHARED);
+
+	if (oldSnapshotControl->count_used > 0
+		&& ts >= oldSnapshotControl->head_timestamp)
+	{
+		int			offset;
+
+		offset = ((ts - oldSnapshotControl->head_timestamp)
+				  / USECS_PER_MINUTE);
+		if (offset > oldSnapshotControl->count_used - 1)
+			offset = oldSnapshotControl->count_used - 1;
+		offset = (oldSnapshotControl->head_offset + offset)
+			% OLD_SNAPSHOT_TIME_MAP_ENTRIES;
+
+		*xlimitp = oldSnapshotControl->xid_by_minute[offset];
+
+		in_mapping = true;
+	}
+
+	LWLockRelease(OldSnapshotTimeMapLock);
+
+	return in_mapping;
+}
+
+/*
  * TransactionIdLimitedForOldSnapshots
  *
- * Apply old snapshot limit, if any.  This is intended to be called for page
- * pruning and table vacuuming, to allow old_snapshot_threshold to override
- * the normal global xmin value.  Actual testing for snapshot too old will be
- * based on whether a snapshot timestamp is prior to the threshold timestamp
- * set in this function.
+ * Apply old snapshot limit.  This is intended to be called for page pruning
+ * and table vacuuming, to allow old_snapshot_threshold to override the normal
+ * global xmin value.  Actual testing for snapshot too old will be based on
+ * whether a snapshot timestamp is prior to the threshold timestamp set in
+ * this function.
+ *
+ * If the limited horizon allows a cleanup action that otherwise would not be
+ * possible, SetOldSnapshotThresholdTimestamp(*limit_ts, *limit_xid) needs to
+ * be called before that cleanup action.
  */
-TransactionId
+bool
 TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
-									Relation relation)
+									Relation relation,
+									TransactionId *limit_xid,
+									TimestampTz *limit_ts)
 {
-	if (TransactionIdIsNormal(recentXmin)
-		&& old_snapshot_threshold >= 0
-		&& RelationAllowsEarlyPruning(relation))
-	{
-		TimestampTz ts = GetSnapshotCurrentTimestamp();
-		TransactionId xlimit = recentXmin;
-		TransactionId latest_xmin;
-		TimestampTz update_ts;
-		bool		same_ts_as_threshold = false;
+	TimestampTz ts;
+	TransactionId xlimit = recentXmin;
+	TransactionId latest_xmin;
+	TimestampTz next_map_update_ts;
+	TransactionId threshold_timestamp;
+	TransactionId threshold_xid;
 
-		SpinLockAcquire(&oldSnapshotControl->mutex_latest_xmin);
-		latest_xmin = oldSnapshotControl->latest_xmin;
-		update_ts = oldSnapshotControl->next_map_update;
-		SpinLockRelease(&oldSnapshotControl->mutex_latest_xmin);
+	Assert(TransactionIdIsNormal(recentXmin));
+	Assert(OldSnapshotThresholdActive());
+	Assert(limit_ts != NULL && limit_xid != NULL);
 
-		/*
-		 * Zero threshold always overrides to latest xmin, if valid.  Without
-		 * some heuristic it will find its own snapshot too old on, for
-		 * example, a simple UPDATE -- which would make it useless for most
-		 * testing, but there is no principled way to ensure that it doesn't
-		 * fail in this way.  Use a five-second delay to try to get useful
-		 * testing behavior, but this may need adjustment.
-		 */
-		if (old_snapshot_threshold == 0)
-		{
-			if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin)
-				&& TransactionIdFollows(latest_xmin, xlimit))
-				xlimit = latest_xmin;
+	if (!RelationAllowsEarlyPruning(relation))
+		return false;
 
-			ts -= 5 * USECS_PER_SEC;
-			SetOldSnapshotThresholdTimestamp(ts, xlimit);
+	ts = GetSnapshotCurrentTimestamp();
 
-			return xlimit;
-		}
+	SpinLockAcquire(&oldSnapshotControl->mutex_latest_xmin);
+	latest_xmin = oldSnapshotControl->latest_xmin;
+	next_map_update_ts = oldSnapshotControl->next_map_update;
+	SpinLockRelease(&oldSnapshotControl->mutex_latest_xmin);
 
+	/*
+	 * Zero threshold always overrides to latest xmin, if valid.  Without some
+	 * heuristic it will find its own snapshot too old on, for example, a
+	 * simple UPDATE -- which would make it useless for most testing, but
+	 * there is no principled way to ensure that it doesn't fail in this way.
+	 * Use a five-second delay to try to get useful testing behavior, but this
+	 * may need adjustment.
+	 */
+	if (old_snapshot_threshold == 0)
+	{
+		if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin)
+			&& TransactionIdFollows(latest_xmin, xlimit))
+			xlimit = latest_xmin;
+
+		ts -= 5 * USECS_PER_SEC;
+	}
+	else
+	{
 		ts = AlignTimestampToMinuteBoundary(ts)
 			- (old_snapshot_threshold * USECS_PER_MINUTE);
 
 		/* Check for fast exit without LW locking. */
 		SpinLockAcquire(&oldSnapshotControl->mutex_threshold);
-		if (ts == oldSnapshotControl->threshold_timestamp)
-		{
-			xlimit = oldSnapshotControl->threshold_xid;
-			same_ts_as_threshold = true;
-		}
+		threshold_timestamp = oldSnapshotControl->threshold_timestamp;
+		threshold_xid = oldSnapshotControl->threshold_xid;
 		SpinLockRelease(&oldSnapshotControl->mutex_threshold);
 
-		if (!same_ts_as_threshold)
+		if (ts == threshold_timestamp)
+		{
+			/*
+			 * Current timestamp is in same bucket as the the last limit that
+			 * was applied. Reuse.
+			 */
+			xlimit = threshold_xid;
+		}
+		else if (ts == next_map_update_ts)
+		{
+			/*
+			 * FIXME: This branch is super iffy - but that should probably
+			 * fixed separately.
+			 */
+			xlimit = latest_xmin;
+		}
+		else if (GetOldSnapshotFromTimeMapping(ts, &xlimit))
 		{
-			if (ts == update_ts)
-			{
-				xlimit = latest_xmin;
-				if (NormalTransactionIdFollows(xlimit, recentXmin))
-					SetOldSnapshotThresholdTimestamp(ts, xlimit);
-			}
-			else
-			{
-				LWLockAcquire(OldSnapshotTimeMapLock, LW_SHARED);
-
-				if (oldSnapshotControl->count_used > 0
-					&& ts >= oldSnapshotControl->head_timestamp)
-				{
-					int			offset;
-
-					offset = ((ts - oldSnapshotControl->head_timestamp)
-							  / USECS_PER_MINUTE);
-					if (offset > oldSnapshotControl->count_used - 1)
-						offset = oldSnapshotControl->count_used - 1;
-					offset = (oldSnapshotControl->head_offset + offset)
-						% OLD_SNAPSHOT_TIME_MAP_ENTRIES;
-					xlimit = oldSnapshotControl->xid_by_minute[offset];
-
-					if (NormalTransactionIdFollows(xlimit, recentXmin))
-						SetOldSnapshotThresholdTimestamp(ts, xlimit);
-				}
-
-				LWLockRelease(OldSnapshotTimeMapLock);
-			}
 		}
 
 		/*
@@ -1867,12 +1879,18 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
 		if (TransactionIdIsNormal(latest_xmin)
 			&& TransactionIdPrecedes(latest_xmin, xlimit))
 			xlimit = latest_xmin;
+	}
+
+	if (TransactionIdIsValid(xlimit) &&
+		TransactionIdFollowsOrEquals(xlimit, recentXmin))
+	{
+		*limit_ts = ts;
+		*limit_xid = xlimit;
 
-		if (NormalTransactionIdFollows(xlimit, recentXmin))
-			return xlimit;
+		return true;
 	}
 
-	return recentXmin;
+	return false;
 }
 
 /*